CuII–PDC-bpe frameworks (PDC = 2,5-pyridinedicarboxylate, bpe = 1,2-di(4-pyridyl)ethylene): mapping of herringbone-type structures

Abstract

Solid coordination frameworks (SCF) are crystalline materials based on connections between metal ions through organic ligands. In this sense, combination of polycarboxylate anions and dipyridyl ligands is an effective strategy to produce extended structures. In this context, this work is focused on two novel Cu^II-based SCFs incorporating PDC (2,5-pyridinedicarboxylate) and bpe (1,2-di(4-pyridyl)ethylene): Cu₂[(PDC)₂(bpe)(H₂O)₂]·3H₂O·DMF (1), and Cu[(PDC)(bpe)_0.5(H₂O)]·2H₂O (2), where DMF is dimethylformamide. Both compounds were synthesized by slow evaporation, and their crystal structures were determined by X-ray diffraction. Further structural, thermal and magnetic characterization was carried out by means of IR, TG/DTG, DTA analysis, EPR, and measurements of the magnetic susceptibility. The crystallographic analysis revealed that compounds 1 and 2 can be described as herringbone-type layers formed by helicoidal Cu–PDC–Cu chains connected through bpe ligands. Solvent molecules are crystallized between the layers, providing the inter-layer connections through hydrogen bonds. Differences between the two compounds are attributable to these solvent molecules, being indicative of the flexibility of this type of SCFs. On the other hand, due to the variety of structures found in the literature that have been described as “herringbone arrays”, this work also presents a crystallochemical study based on them. The study considers stoichiometry and structural parameters leading to the identification of two types of herringbone arrays, depending on the number of connections for the metal nodes (i.e. 3- and 4-connected).